Control system for roll-table motors



Nov. 7,- 1933. RIM. KENAH 1,934,505

CONTROL SYSTEM FOR ROLL TABLE MOTORS Filed March 2, 1931 2 Sheets-Sheet1 lV/f/VESSES:

Nov. 7, 1933.

R. M. KENAH 1,934,505

CONTROL SYSTEM FOR ROLL TABLE MOTORS Filed March 2, 1931 2 Sheets-Sheet2 flax/7mm Speed afredaced wllage 00d fi'eyaency A Zero Speed Zero SpeedE95. 8 A Zero E B AZam lkx/rmm Speed of reduced l o/fage arid fieyueflcgSpaed a7 l a/iaye and F/equency IV/TA/ESSES; INVENTOR 50/0/7077.ffena/z.

' 7ATT6RNEY Patented Nov. 7, 1933.

CONTROL. SYSTEM FOR ROLL-TABLE MOTORS Roland M. Kenah, Pittsburgh, P82",asslguor to Westinghouse Electric & Manufacturing Company, a corporationof Pennsylvtinia Application March 2, 1931;, Serial No, 519,473

Ill Claims. (ill. lift- EW) tor connected to all the roll members of thetable by means of a lay shaft and suitable bevel gears.

In some instances, however, roll tables have been operated by individualmotors, preferably of the induction type, connected to each roll of thetable and all of the motors controlled, as a unit, from a singlegenerator by varying the generator voltage.

However, in using the latter type of drive, considerable diiiiculty hasbeen experienced in preventing injury to the roll motors fromoverheating, since their location naturally requires that they shall bealmost entirely enclosed to prevent mechanical injury and to excludedust, scale and other foreign matter which is found in steel mills ingreat quantities.

When the motors are placed under such edverse operating conditions, itbecomes very dilficult to operate them in such manner as to maintaintheir windings at safe temperature, since they are not only heated up bythe high tenn perature of the air in their immediate vicinity caused bythe hot metal which is carried on the roll table but also by the highcurrents which flow in their windings during the starting and pluggingperiods which may occur quite frequently where it is necessary to startand stop the roll table at frequent intervals.

Accordingly, the object of my invention, gen erally stated, is toprovide a motor-control system which shall be simple and efiicient inoperation and easy and economical to manufacture and install. a

A more specific object of my invention is to provide forso effecting theaccelerating end plugging operation of motors operating under adversetemperature conditions as to prevent over-l1eating and consequent injuryto the windings.

Another object of my invention is to provide for controlling theoperation of a plurality of roll-table motors by varying the voltage andfrequency of the current supplied to them from a single generator toreduce the heating eilect of the starting and plugging currents to amini mum value. I

A, still further object of my invention is to an" tomatically delayestablishment of plugging connections for the roll-table motors untilthe volt age and frequency of the generator, supplying current to themotors, have been reduced to predetermined values.

Other objects of my invention will become evident from the followingdescription, taken in conjunction with the accompanying drawings inwhich,

Figure l is a diagrammatic view of a motor control system embodying theprincipal features of my invention, and

Figs. 2 to 5, inclusive, are diagrams showing how the heating effect ofthe starting and plugging current may be materially reduced by the useof my invention.

Referring now to Fig. l of the drawings, 10 dos-- ignates, generally, aroll table of conventional design having a plurality of roll members 11,all of which are disposed to be rotated at the same speed to conveymetal in the form oi ingots or billets, to and from a rolling mill. inorder to operate the rolls ll, I provide an individual motor 12 for eachroll which may be connected to its roll in any suitable manner. Theseauxiliary or roll motors may be of any suitable type, but, in

"this embodiment of the invention, three-phase induction motors of thesquirrel-cage type are utilised.

The roll motors 12 may be supplied with current from any suitable powersource such, for ex ample, as a motor-generator set l3, comprising analternating-current generator 14; and a directcurrent driving motor 15,having their rotating" elements mechanically connected, by means a shaft16', in he usual manner.

Power for operating the motongenerator set 13, as well the variouscontrol devices of e system, may he obtained from. any suitaiile sourceor substantially constant voltage, direct current power which, ininstance, is iilus trated by conductors l7 and. i8.

its will he observed, the roll meters 12 versed, in order? to energizethe roll motors for reverse operation and may, therefore, be termed thereversing line switch or plugging switch, since, in this embodiment ofthe invention, actual reversed running operation of the roll motors isnot eflected, as will be more clearly set forth hereinafter.

, The system may be controlled, and especially the operation of the lineswitches 22 and 23, by any suitable control device, however, in thisembodiment of the invention, a manually-operated master switch, showngenerally at 24, is utilized. As shown, the master switch comprisesmovable segments 25 and 26, each of which is disposed, when actuated inthe forward running and plugging directions, respectively, to engagecertain of the fixed contact fingers a, b, c and d.

In order that the novel features of my invention may be more readilyunderstood, a detailed description of the operation of the system willnow be given.

In applications to which my invention is particularly adapted, it is theusual practice to start and stop the roll table frequently. Since thestarting and stopping operation must be effected within the shortesttime possible, provision must be made for accelerating and retarding theroll motors in a rapid manner and still maintain the temperatures of themotors at the lowest possible value, even though they'may be required tooperate under the most adverse conditions, insofar as atmospherictemperature is concerned.

Therefore, in order to provide for quickly accelerating the roll motorsand, at the same time, maintain their temperatures at the lowestpossible value, provision is made for initially subjecting them to avoltage and frequency of reduced value, preferably from twenty-five tofifty percent of the full voltage and frequency value. If it is desired,the reduced voltage may be made slightly greater, in proportion, thanthe reduced frequency, in order to stabilize the voltage of thegenerator when the load is suddenly applied, as when in starting, and tocause the roll motors to develop a greater starting torque.

'I'he motor-generator set 13 is first set in operation by actuating themanual controller 27 to the first operating position to bridge contactfingers e and j and thereby establish an energizing circuit for lineswitches 28 and 29, as shown. Upon the closure of switches 28 and 29,the direct current motor 15 is connected across the line conductors 17and 18 through the starting resistor 31.

The speed of the motor-generator set may be further increased byoperation of the manual controller 27 to the second position wherecontact fingers e and g are bridged to establish an obvious energizingcircuit for an accelerating switch 32.

As will be observed, the motor 15 is provided with a shunt field winding33, connected in series with a variable resistor 34 which may beadjusted to cause the motor 15 to operate at any desired speed. i

In order, however, to cause the generator to develop a reduced voltageand frequency, provision is made for subjecting the motor 15 to 1111field excitation until the roll motors have been connected to thegenerator and have had sufhcient timeto accelerate to a speedproportional to the reduced voltage and frequency of the generator. Thisfunction is accomplished by the use of a switch 35 having contactmembers 36 which are closed when the switch is deenergized. At thistime, the switch 35 is deenergized, since the I connection betweenconductors 19 and 21 remaster controller 24 is in the on position, and,therefore, the contact members 35 establish a shunt circuit for thefield resistor 34 to impress full voltage of the line conductors 1'7 and18 across the shunt field winding 33.

As will be readily understood, the generator 14 is now operating atreduced speed and, consequently, it can develop only a certain voltageand frequency with a given amount of field excitation. As shown, thefleld winding 3'? of the generator is connected across the lineconductors 1'7 and 18 through a variable resistor 38, which may beadjusted to cause the generator to develop the (18- sired voltage.

If it now be desired to set the roll-table motors in operation, masterswitch 24 may be actuated to the first position in the forward-runningdirection, which causes a movable segment 25 to bridge contact fingers aand b. This establishes an energizing circuit for theforward-running'switch 22 which extends from line conductor 18, throughconductor 39, contact fingers a and b, conductor 41 of master switch 24,bridged by segment 25 and the operating coil of the switch 22, con"ductor 56 to the line conductor 17. Upon the closure of the switch 22,the roll motors are connected to the generator 14 in such phase relationas to cause them to run in a forward direction, and, since the voltageand frequency of the generator 14 are of reduced value, the roll motorswill quickly accelerate to a speed proportional to the voltage andfrequency values.

In order to further increase the speed of the roll motors 12 to theirnormal value, provision is made for increasing the speed of themotorgenerator set 13 to such value as to cause the generator 14 todevelop full voltage and fre quency, thereby causing the roll motors tocome up to maximum speed in accordance with the rise of generatorvoltage and frequency.

In this embodiment of the invention, this function is effected bydecreasing the field excitation of the motor 15 through the operation ofthe shunting switch 35 which may be energized by actuating the masterswitch 24 to the second position in the forward-running direction. Asshown, the circuit thus established extends from line conductor 18,conductor 39, through contact fingers a and cbridged by the segment 25of the master switch 24-conductor 42, operating coil of the switch 35and conductor 43, to line conductor 1'7.

As described, in the preceding paragraph the operation of the shuntingswitch 35 to control the field excitation of the motor 15 is controlleddirectly through the master switch 24 which does not definitely providefor a time interval, between the operation of switches 22 and 35, ofsuflicient length to permit the roll motors to accelerate fromstand-still to the reduced speed value before the speed of the motorgenerator set is increased, except that which is introduced by theoperator in actuating the master switch.

It will be readily understood, however, that, if it were thought to bedesirable or necessary, a suitable time-delay relay might be readilyinterposed between the master switch 24 and the switch 35 in order toprovide this definite time interval between the operation of theswitches 22 and 35.

In order to quickly stop the roll motors after they have been operatingin a forward direction for the desired length of time, provision is madefor plugging the motors in such manner as to effect a quick stoppingoperation and, at the same permanently connected across the lineconductors time, very materially reduce the heating eflect ordinarilycaused by plugging operations.

In order to accomplish the foregoing purpose, provision is made fordefinitely maintaining the forward-running switch 22 in the closedposition, regardless of the position of the master switch 24, untilthespeed of the motor-generator set 13 has been reduced to its initialvalue, where the generator only develops voltage and frequency atreduced value.

The speed of the motor-generator set 13 is reduced when the masterswitch 24 is actuated in the reverse direction from its last or secondposition in the forward-running direction which deenergizes the switch35 to subject the motor 15 to full field excitation, as describedhereinbefore. Therefore, it is evident that, as soon as the masterswitch 24 is moved from this second position, the motor-generator set 13begins to slow down, with the roll motors still connected for forwardrunning, which causes them to likewise reduce their speed in accordancewith the changes in voltage and frequency or the generator. Thisprovides for quickly reducing the speed of the roll motors to a low orreduced value where they may be finally stopped by a plugging operation.

In this embodiment of the invention, the forward-running connection ofthe roll motors is maintained by means of a time-delay relay 44 whichfunctions to maintain an energizing circuit for the forward-runningswitch 22 for a predetermined time interval after the master switch hasbeen actuated to the oil or plugging positions. Any suitable type oftime-delay relay may be used for this purpose, but, in this instance, Ihave illustrated a particular type of relay which is fully described inLetters Patent No. 1,753,983 granted to W. G. Cook, April 8, 1930.

Since the mechanical structure and principle of operation of thetime-delay relay 44 are fully described in the patent designated in thepreceding paragraph, it has been deemed unnecessary to set forthadetailed description in this specification. However, in a brief manner,it may be said that the relay is provided with magnetizing coils 45 and46, both of which must be energized in order to cause the armature .47to close the contact members 48.. As shown, the coil 45 is 17 and 18,while the energization of the coil 46 is controlled by auxiliary contactmembers 51 provided on the shunting switch 35 which, when closed,connect the coil 46 across line conductors 1'1 and 18 in parallel with aresistor 52. When the coil 46 is deenergized, it discharges through alocal or discharge circuit formed by resistor 52 to control the timedelay in the opening of armature 4'7 to effect disengagement of contactmembers 48.

Assuming that the master switch 24 has been actuated to the firstposition in the plugging direction, an energizing circuit for thereverserunning or plugging switch 23 is partially established from theline conductor 18, through'condoctor 39, contact fingers a and d-bridgedby segment 26-and conductor 53, to back contact members 54 of the switch22, which are now open, since the switch isheldclosed.

As soon as the time' delay relay 44 opens its contact members 48, theholding circuit for switch 22 is interrupted which" causes the switch toopen the forward-running connection between the roll motors and thegenerator and, at the same time, close its back contact members 54, tocomplete the partially established energizing circult for thereverse-running or plugging switch 23, through conductors 55 and 56. toline conductor 1'1, as shown.

As will be readily understood, the closure of switch 23 reverses theconnection of the roll motors to the generator 14, while the motors arestill running in a forward direction at reduced speed, therebyperforming what is commonly known as a plugging operation, which quicklyreduces the speed of the roll motors to zero value, whereupon the masterswitch 24 may be returned to the off position.

It will be apparent from the foregoing description that my'inventionprovides for always starting and plugging the roll motors at reducedvoltage and frequency and that, by the use of such a system as providedby my invention, the roll motors may be operated under the most adverseoperating conditions from both the mechanical and temperaturestandpoint. By reducing the heating effect of the starting and pluggingcurrent to a minimum, motors of standard or ordinary construction may beused to operate a roll table, thereby making it unnecessary to usemotors specially constructed to withstand high operating temperatures.

The manner in which my invention provides for reducing the heatingefiect of the starting and plugging currents maybe more fully understoodby considening the diagrams shown in Figs. 2 to 5.

Fig. 2 shows, graphically, the conditions existing in a motor of theinduction type when it is connected to full generator voltage andirequency, at standstill, and accelerated to synchronous or normalspeed. The energy stored in the rotating parts of the motor and load isrepresented by the area A, B, C. The secondary loss under suchconditions is represented by the shaded area A, C, D. Fig. 3 shows theconditions 1 existing in the same motor when started on voltage andfrequency of reduced value, preferably about one half the maximum valuefor full speed operation. It will be observed that the shaded area A, C,D, in Fig. 3, which represents the sec- 1 ondary losses is only aboutone quarter in size to that of Fig. 2, when the motor is started onmaximum voltage and frequency.

Fig. 4 shows the conditions in the same motor when it is plugged, onfull voltage and frequency, from maximum running speed to standstill,while Fig. 5 shows the conditions existing in the same motor when it isplugged, on reduced generator voltage and frequency, from approximatelyhalf speed to standstill. The secondary losses 1 in both cases areindicated by the shaded areas AEDC. As in the case of the startingoperation, the secondary losses when the motor is plugged at reducedvoltage and frequency are about one quarter of the losses which resultwhen the same 1 motor is plugged on full voltage and frequency.

In the foregoing discussion of the motor losses, only the secondarylosses have been considered. These losses, which occur during thestarting period, are fixed by the inertia of the moving connected duringthe starting operation and duning the period before they have beendisconexperience has demonstrated that the total tern-v perature lossduring one cycle 0! operation or the system according to my invention isonly about one-third of the losses which result when the system isoperating constantly on full voltage and frequency. 1

It may be stated, in conclusion, that, while the illustrated exampleconstitutes a practical embodiment of my invention, I do not wish tolimit myself strictly to theexact details herein illustrated,since'modtilcations of the same may be made without departing from thespirit of the invention, as defined in the appended claims.

I claim as my invention:

1. In a motor-control system, in combination, a work motor, a generatorfor supplying power of variable voltage and frequency for said motor,

means for driving the generator at variablespeeds, means for causing thegenerator-driving means to drive the generator at such speed as todevelop a voltage of low value and frequency, means initially operableto connect the work motor to the power source to accelerate the motor toa speed corresponding'to the voltage and frequency of the power source,and means, dependent upon the further operation 01' said last namedmeans, for increasing the speed of the generator driving means andthereby the voltage and irequency of the generator to eifect theacceleration of the work motor to full speed, whereby the work motor isstarted and accelerated to full speed with minimum heating.

2. In a motor-control system, in combination, a plurality of workmotors, a motongenrrator set for supplying power to said motors, asiurce of power for the motor-generator set, means for causing themotol-generator set to opera e at reduced speed, a master switch, meansresponsive to the operation of the master switch for connecting the workmotors to the generator in group relation to accelerate the motors to aspeed corresponding to the generator speed, and means responsive tofurther. operation of the master switch for increasing the speed of themotor-generator set to maximum, whereby the work motors are acceleratedto full speedfrom the reduced speed in accordance with the gradualincrease in generator voltage, thereby to reduce their temperature riseto a minimum.

3. In a motor-control system, in combination, a plurality of workmotors, a motor-generator set for supplying power to said motors, asource of power for the motor-generator set, means for causing themotor-generator set to operate at reduced speed, a master switch, meansresponsive to the operation oi! the master switch for connecting thework motors to the generator in group relation to accelerate the motorsto a speed corresponding to the generator speed, and means dependentupon the further operation of the masster switch for increasing thevoltage and frequency of the generator, thereby to cause the work motorsto reach full-speed operation with a minimum rise of internaltemperature.

. 4. In a motor-control'system, in combination, a motor, amotor-generator set for supplying variable-voltage alternating currentpower to the motor, a source of power, means for connecting the motor ofthe motor-generator set to the source of power to cause the generator todevelop a voltage of predetermined value and frequency, a master switchdisposed to be actuated to a plurality of positions, means responsive tothe operation oi! the master switch to one position to connect the motorto the generator to accelerate the motor to a speed proportional to thevoltage and frequency of the generator, and means disposed to respond tothe further operation of the master switch for increasing the speed ofthe motor-generator set to cause the motor to increase its speed tomaximum value in accordance with. the rise in voltage and frequency ofthe generator.

5. In a motor-control system, in combination, a motor, a motor-generatorset for supplying variable-voltage alternating-current power to themotor, a source of power, means for connecting the motor 0! themotor-generator set to the source of power to cause the generator todevelop a voltage of predetermined value and frequency, a master switchdisposed to be actuated to a plurality of positions, means responsive tosuccessive operation of the master switch for first connecting the motorto the generator and subsequently increasing the speed of themotorgenerator set to cause the motor to operate at full speed, andmeans responsive to the operation of the master switch in the oppositedirection for causing the speed of the motor-generator set to be reducedto the initial speed and for reversing the connection of the motor tothe generator after a predetermined time interval to effect pluggingoperations of the motor only when its speed has been reduced to a valuecorresponding to the initial voltage and frequency of the generator.

6. In a motor-control system, in combination, a plurality oi inductionmotors disposed to opcrate as a group, an alternating-current generatorfor supplying power at variable voltage and. frequency to the inductionmotors, a motor for driving the generator, a source of power of saidmotor, means for connecting the motor to the power source to obtainoperation of the genorator at reduced speed, reversing switches forconnecting the induction motors to the generator, 8. master switch forcontrolling the operation of the reversing switches, said master switchbeing disposed, when actuated to successive positions in one direction,to first connect the induction motors to the generator while operatingat reduced speed and to subsequently increase the voltage and frequencyof the generator to full value, thereby to effect the acceleration ofthe induction motors to full speed with a minimum or heat losses.

7. In a motor-control system, in combination, a plurality of motorsdisposed to be operated as a group, a motor-generator set for supplyingpower to the group of motors, means for eifecting the operation of themotor-generator set at reduced speed, means including a master switchfor first connecting the motors to the generator and for subsequentlyeffecting an increase in speed or the motor-generator set, and meansdependent upon the operation of the master switch in the oppositedirection for first reducing the speed 0! the motor-generator set andfor subsequently reversing the connection of the group of motors to thegenerator a predetermined time interval after the operation or themaster switch from the initial position to the opposite positiom 8. Thecombination with a plurality of induction motors and a motorgeneratorset for supplying variable voltage and frequency power,

to the motors, of means for effecting the operation of the motorgenerator set at a predetermined speed less than maximum, control meansoperable to connect the induction motors to the generator to effecttheir energization in either direction, means operable to increase thespeed of the motor-generator set, said means being responsive to theoperation of the control means to a predetermined position in onedirection, and means rendered effective by the operation of the controlmeans in the opposite direction to delay the reversal of the generatorconnection to the induction motors until the motor-generator set hasreduced its speed to the initial value less that maximum, thereby toreduce the time during which a heavy current flows in each inductionmotor during the decelerating period to a minimum value.

- subsequent to the energization of the forwardrunning connecting switchfor increasing the speed of the generator-driving motor to cause thegenerator to develop full voltage, and a timedelay' relay renderedeflective by the subsequent operation of the master switch to theplugging position for maintaining the forward running connection betweenthe work motors and the generator until the generator voltage hasreturned to the original low value, thereby to provide for plugging thework motors only on low voltage.

10. In a motor-control system, in combination, a plurality of inductionmotors, an altematingcurrent generator for supplying power to theinduction motors, reversing switches for connecting the induction motorsto the generator, a motor provided with a field winding for driving thegenerator, a source of power for said motor, a variable resistor forcontrolling the excitation of the motor, a normally closed shuntingswitch for the resistor, means operable to connect the generator-drivingmotor to the power source, with its field winding fully energized tocause the generator to develop a voltage and frequency less thanmaximum, a master switch operable in one direction to first efiect theclosure of one reversing switch to connect the induction motors foroperation in a forward direction and for sub- 100 sequently efiectingthe opening of the resistorshuntlng switch to increase the speed of thegenerator to maximum, said master switch being disposed, when actuatedin the opposite direction. to partially establish an energizing circuitfor 105 the other reversing switch, and 'means rendered efiective uponthe closing of the i'esistor-shunting switch for delaying the finalenergization of said other reversing switch until the voltage andfrequency of the generator has been reduced to 0 the initial value.

ROLAND M. KENAH.

